This invention relates to cooking ovens. In particular, it relates to ovens for forced convection cooking with superheated steam, saturated steam, or heated air, all of which are heated by the combustion of gas.
The earliest gas-fired oven could be described as a cube within a cube. The inner and outer cubes shared a common front vertical face which contained a door for access to the inner cooking compartment. Gas is burned in the space between the bottoms of the inner and outer cubes. Combustion products transferred heat to the inner compartment in their upward travel between the sides and backs of the cubes, and were exhausted via natural draft through holes in the top of the outer cube. Objects to be cooked were placed in the inner cube. These simply designed ovens typically suffered from low efficiency of heat exchange, scorching of the food in the bottom of the cooking compartment directly above the burners, and severe temperature stratification in the air space within the cooking compartment.
In transferring heat from a heat source such as burning gas to a quiescent gaseous fluid medium such as air, and finally to an object for cooking, the free-convection coefficient for heat transfer from an air film to food is the main limit on the overall rate of heat transfer. By employing a fan to circulate the heated air within the cooking chamber, and hence transfer heat from the air to the food through forced convection, it was found that cooking speed could be significantly increased. Through air circulation, these so-called convection ovens also exhibited greatly improved temperature uniformity within the cooking compartment.
A subsequent modification to gas-fired convention ovens allowed the gas combustion products to pass through the cooking compartment and directly transfer heat to the food. The convection fan draws the flue gases through holes in the top of the cooking chamber, circulates them about the cooking chamber, and then exhausts the gases through other holes in the top of the oven. This essentially reduced the exit temperature of the flue gas to that of the oven cooking chamber, and thus increased the efficiency of heat exchange. In reference to the vertical standpipe through which flue gases were typically drawn from above the cooking compartment and down into the fan inlet, these modified gas convection ovens were called snorkel ovens.
Modern commercial gas ovens are designed either with and without convection fans, and either with and without passing flue gases through the cooking chamber. The principle of the gas snorkel convection oven, applied in various detailed spatial configurations, represents the current state of the art in commercial gas ovens.
The art of steam cooking was developed as an alternative to cooking food in a free- or forced-convection hot air environment. Surface heat transfer coefficients associated with condensing saturated steam are typically greater than those of circulating hot air. Foods cooked in steam cook more quickly, lose less moisture, keep their food values and look better. Conventional steamers surround food objects with a continuous supply of saturated steam. This is usually done at atmospheric pressure, but saturated steam at elevated temperatures can be obtained in a pressure cooker.
In atmospheric steamers, a condensate drain is placed in the bottom of the cooking compartment to remove liquid water which condenses on the compartment surfaces and on the food object and to carry away any entrained liquids originating from the food. With the exception of the condensate drain and the steam inlet port, the atmospheric steamer compartment represents a closed system. Unlike conventional dry convection ovens, a tight door seal is achieved with a gasket of silicone rubber or the like. By continuously supplying the compartment with steam at atmospheric pressure, all air is flushed from the compartment. Cooking takes place in an environment of saturated steam in the absence of dilution air. In an atmospheric convection steamer, the saturated steam is circulated within the compartment by a fan.
Atmospheric steamers can be either gas-fired or powered by electricity. To provide the required steam throughput, gas-fired steamers typically employ relatively large gas-fired tube boilers positioned beneath the steamer compartment. The large surface area for heat transfer between the fire tube and water and the resulting large volume of water required in a conventional gas-fired steam generator typically prevents the use of gas-fired steamers as integral counter-top units.
Extraordinary cooking performance beyond the capabilities of separate convection ovens or steamers has more recently resulted from combining the functions of an atmospheric steamer and a convection oven into a single unit. These versatile modern combination convection oven-steamers, or combination ovens, exhibit increased cooking speed at reduced oven temperature, reduced food moisture loss, browning capabilities of dry convection ovens, and the ability to control relative humidity as well as temperature within the cooking chamber. The combination cooking mode involves providing the oven cavity with a continuous supply of saturated steam at atmospheric pressure, and maintaining an environment of circulating superheated steam in the oven cavity by employing the normal oven heat source, thermostat, and convection fan. The combination oven can also function as a dry convection oven or an atmospheric convection steamer. As with the atmospheric steamers described earlier, combination ovens are equipped with a condensate drain and a tight door seal. Combination convection oven-steamers represent the current state of the art in commercial cooking and their broad potential for specific cooking applications has not yet been fully tapped.
Available evidence to date suggests that the widely heralded benefits of combination-mode cooking depend on the maintenance of a circulating superheated steam environment in the absence of dilution air. In transferring heat from condensing steam to a solid surface, it is well known that dilution air can form an insulating barrier on the surface of the solid object which inhibits heat transfer from the condensing steam. All or nearly all commercially feasible combination ovens are currently powered by electricity, and all combination ovens currently produced in the U.S. are exclusively electric powered. As with the atmospheric steamer described earlier, electric combination ovens can effectively prevent dilution air from contaminating the superheated steam oven. On the other hand, because the oven cavity more completely resembles a true open system, a conventional gas-fired snorkel convection oven cannot be directly applied to maintain a circulating environment of superheated steam in the absence of dilution air, even by incorporating a vent damper.
The major challenge in developing a successful gas-fired combination convection oven-steamer, then, is to design a gas-fired oven heat exchanger which is compact, efficient, reliable, cost-effective, and, most importantly, which can maintain a circulating environment of undiluted superheated steam with precise temperature control in space and time. A second challenge in developing an attractive gas-fired combination oven is to design a compact, efficient steam generator which is physically configured so as to allow counter-top implementation of the unit by eliminating the large space requirements associated with conventional gas-fired steam generators.
A further problem in cooking arises from the fact that cooking cycles sometimes require temperature changes. In such a case, it is necessary to have some means of storing a desired temperature control level for a particular time and a different temperature control level for a different period of time. Information such as this is best handled by a microprocessor with associated memories.